outside the shielded room (to eliminate 50-Hz ac power line noise from the room). The output signal from the SQUID magnetometer is passed through a 100-Hz low-pass filter and a 50-Hz notch filter, and is recorded on a chart recorder. Fig. 7 shows a simultaneous recording of the magnetocardiogram and the electrocardiogram.The magnetometer was calibrated, and the peak height of the magnetocardiogram was found to be 0.5 to 1 X 10~1 0 T, and the noise level was 5 X 10~1 2 T. The noise level is five times as large as the SQUID flux resolution, which is 3.3 X 10~1 9 Wb/Vîîz. The baseline noise level is affected by vibration of the shielded room and SQUID probe.
VII. C O N C L U S I O N SA new niobium thin-film SQUID has"been developed. The thin film is deposited on a sapphire substrate using RF sput tering. A microbridge is fabricated using photolithography and sputter etching, and its surface is oxidized. This SQUID is suitable for mass production and has excellent mechanical stability. The thin-film SQUID was mounted on a niobium block to reduce the SQUID's ring inductance and increase its sensitivity. The SQUID operated in the temperature range from 4.1 to 4.3 K and had an energy resolution better than 5 X 10" 2 9 J/Hz.A SQUID magnetometer incorporating this SQUID had current sensitivity of 0.16 μΑ/Φο· The magnetometer was used with gradiometer-type search coils to measure magnetocardiograms in a magnetically shielded room. Experiments suggest that, by using more accurate second-derivative gra diometer-type search coils, it will be practicable in the future to measure magnetocardiograms without using a shielded room.Abstract-A new system for measuring low dc current covering a wide range of currents is presented. It eliminates the need of switches in the circuit where the measured current flows. The system design simplifies the ranging problem and paves the way for easier develop ment of instrumentation where low dc currents should be measured. , "Design and operation of stable rf-biased superconducting point contact quantum devices, and a note on the properties of perfectly clean metal contacts,"
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